You need to check the door springs and the drive gear in the opener.
Garage door opener why the door slide back down.
We recommend unplugging the garage door opener and then replugging it back in after any repairs or adjustments to reset the system.
Check to be sure these two sensors on either side of your garage door.
It will be obvious if it is going bad.
You will have to take the cover off the motor unit to check the gear.
The one that is not is not strong enough to hold door up on its own but strong enough to still open the door.
Thank you for your question as to why the garage door will open all the way but slides back 6 inches.
Door sliding back down once the door is up all the way it s resting on the horizontal parts of the track and the resistance of the motor to turning because it is low geared should prevent it coming back down.
It can also be caused by dirt on the sensor eyes loose or corroded wire connections at the sensors or the motor head or just that the sensors have gone bad and need to be replaced.